Introduction to RTOS Part 8 - Software Timer | Digi-Key Electronics

Summary

This video explains how to use timers in FreeRTOS on the ESP32, emphasizing their importance for scheduling periodic tasks like blinking LEDs, polling sensors, or refreshing displays. It covers different timer types, such as one-shot and auto-reload timers, and demonstrates how to implement them in an efficient manner with minimal overhead. The tutorial also highlights best practices for handling callback functions and introduces a practical use case: controlling an LCD backlight with a timer that dims after a period of inactivity. A great resource for those looking to optimize task management in embedded systems.

Takeaways

• 😀 Timers are valuable tools for delaying or periodically executing functions in programming, particularly in microcontrollers.
• 🕒 Microcontrollers often rely on software timers for periodic tasks like blinking LEDs or polling sensors instead of using hardware timers, which are limited in number.
• 🛠️ Using tasks and the vTaskDelay function in FreeRTOS allows periodic execution, but this comes with overhead, especially when creating tasks for each function.
• ⏱️ The xTaskGetTickCount function can track elapsed time and manage periodic tasks within a single task, providing a more efficient alternative to creating multiple tasks.
• ⚙️ For higher precision than one millisecond, hardware timers may be needed, but they require specific configuration and limit portability.
• 🖥️ FreeRTOS offers software timers, which execute callback functions when they expire, useful for tasks requiring periodic execution without creating multiple tasks.
• 🔄 Software timers in FreeRTOS operate in the background through the Timer Service Task (or Timer Daemon), which wakes up to execute timers when needed.
• 🚀 Timer callback functions in FreeRTOS should be fast and non-blocking, similar to interrupt service routines, to avoid delays in timer operation.
• ⚠️ Using synchronization mechanisms like queues and mutexes inside timer callbacks is discouraged because they may block the Timer Service Task and introduce delays.
• 📝 FreeRTOS allows configuration of the Timer Service Task, including priority, stack size, and queue length, through the esp32-specific configuration file (freertos_config.h).
• 💡 In the demo, the script showcases both one-shot and auto-reload timers, explaining their differences, including handling periodic versus single-use execution scenarios.
• 🔋 A challenge is presented where the user must use software timers to control an LED's state, mimicking an LCD's auto-dim feature with a delay after user input activity.

Q & A

What is the primary use of timers in microcontroller programming?

Timers are used in microcontroller programming to delay the execution of a function or execute a function periodically. They are essential for tasks like blinking an LED, refreshing an LCD, polling sensors, or sending pulses to devices like servos.

Why might a hardware timer not always be the best solution for managing periodic tasks in a microcontroller?

A hardware timer might not be ideal because microcontrollers have a limited number of these timers available, and using them can make the code less portable. Additionally, configuring and handling hardware timers often requires platform-specific code.

What are the main differences between using FreeRTOS software timers and hardware timers?

Software timers in FreeRTOS are easier to manage at the operating system level, using the timer service task to handle multiple timers with a single task. In contrast, hardware timers offer more precise timing but are more limited and require specific setup for each platform.

How does FreeRTOS handle software timers at the task level?

FreeRTOS uses a background task known as the timer service task (or daemon), which maintains a list of timers and triggers callback functions when the timers expire. This task wakes up only when a timer needs to expire, making it efficient in terms of CPU usage.

What is the role of the timer service task in FreeRTOS?

The timer service task in FreeRTOS runs in the background, managing software timers. It blocks itself until the tick timer reaches a specified time, then triggers the corresponding callback function. This design ensures that the timer handling does not consume unnecessary CPU resources.

Why should timer callback functions be kept quick and non-blocking in FreeRTOS?

Timer callback functions should be quick and non-blocking because they run at the same priority as the timer service task, which is responsible for managing all timers. If these callbacks block, they could delay the execution of other tasks, negatively impacting system performance.

How does the `xTaskGetTickCount` function help manage periodic tasks in FreeRTOS?

The `xTaskGetTickCount` function returns the number of ticks that have passed since the scheduler started, allowing tasks to compare elapsed time and determine when to execute periodic functions. This method works well for simpler tasks without needing dedicated hardware timers.

What configuration changes are required to enable timer support in FreeRTOS for the ESP32?

To enable timer support in FreeRTOS for the ESP32, you need to modify the `FreeRTOSConfig.h` file and set the `configUSE_TIMERS` setting to 1. This is typically enabled by default in the ESP32's FreeRTOS configuration, but can be adjusted as needed.

What is the purpose of the `pdTRUE` and `pdFALSE` flags in FreeRTOS timers?

The `pdTRUE` flag is used to create auto-reload timers, meaning the timer will automatically restart and trigger its callback after each expiry. The `pdFALSE` flag creates one-shot timers, which only trigger their callback function once.

How does the `xTimerStart` function interact with software timers in FreeRTOS?

The `xTimerStart` function is used to start or restart a timer in FreeRTOS. It accepts a timer handle and a timeout period. If the timer is already running, calling this function before expiration restarts the timer, making it useful for managing periodic tasks that require regular execution.